U.S. Pat. No. 4,664,971 discloses a plastic article containing electrically conductive fibres with shielding properties against electromagnetic interference and against radio-frequency. In order to obtain such an article, composite grains or granules with electrically conductive fibres are used as intermediary product. These composite grains are mixed with a predetermined amount of substantially pure plastic material and this blend is introduced in the hopper of for example an extrusion mixture. The material is then heated to soften the plastic and worked to distribute the fibres in the plastic to obtain a plastic article with good shielding properties. U.S. Pat. No. 4,664,971 indicates that blowing agents can be used to manufacture articles of thermoplastic or thermosetting foam material.
U.S. Pat. No. 5,399,295 discloses a composite sheet comprising a continuous matrix of a synthetic resinous material having randomly dispersed therein conductive fibers and a particulate, conductive or semi-conductive filler. Injection moulding is a possible production technique and blowing or foaming agents can be used in the manufacturing of such a composite sheet.
Plastic articles obtained according to U.S. Pat. No. 4,664,971 have acceptable shielding properties in their central zones. The border zones which are the zones close to the surface of the plastic article, however, exhibit a zone rich in pure plastic material and poor in fibre material. In addition, the fibres tend to take a direction parallel to the surface, thereby no longer forming good electrical contact with the outside. Generally there are also almost no protruding fibres through the surface. A housing is generally assembled out of two or more shells that neatly match together. Having regard to the absence of fibres at the surface of the plastic articles, the zone of overlap exhibits a poor concentration of fibres. Electromagnetic radiation looks for the weakest parts. Since the degree of shielding of a complete housing is determined by the parts that are weakest, the zone of overlap is characteristic for the shielding of the complete housing. And since this zone of overlap is poor in shielding, the complete housing is poor in shielding.
The above problem of plastic rich border zones has already been noticed in the prior art, e.g. in EP 0,674,326 describing an electrical conductor comprising an insulation of plastic material. An integral body is made out of a mixture of plastic material and a content of metallic fibers. The fibers have a small cross-sectional dimension relative to the length. The body is moulded by injection moulding or extrusion, respectively, such that during the moulding process the metallic fibers substantially concentrate in the central portion in view of the flowing direction of the moulding process whereby a well-conductive conductor portion is formed.
WO-A-2006/128741 solves this problem by providing an additional piece which protrudes from the eventual surface and afterwards breaking this additional piece away. Although providing an adequate technical solution to the problem, this technique has failed from a cost perspective point of view in a number of applications.
Other techniques avoid the use of electrically conductive fibres and provide the surface of the plastic articles with an electrically conductive coating. These techniques may also be very effective but require the additional step of coating.